Particulate Fuel Combustion Process and Furnace

ABSTRACT

The present invention provides a process for operating a combustion furnace, wherein the combustion furnace comprises a combustion chamber defining a combustion zone within the combustion chamber and having at least one chamber wall facing the combustion zone, and at least one particulate fuel burner mounted in a chamber wall and adapted to generate a flame in the combustion zone by injecting oxidant in gaseous form and fuel in particulate form into the combustion zone for combustion therein. In the process, at least one particulate fuel burner is operated so as to alternate between a first phase in which the burner generates a wide flame proximate the chamber wall into which the burner is mounted and a second phase in which the burner generates a narrower flame directed away from the chamber wall into which the burner is mounted.

CROSS-REFERENCE TO RELATED APPLICATIONS

None.

BACKGROUND

1. Field of the Invention

The present invention relates to the field of particulate fuel combustion and the use of particulate fuel burners in industrial combustion furnaces.

The present invention relates in particular to the use of particulate solid fuel burners, such as particulate coal burners.

The present invention also relates to the use of particulate liquid fuel burners, whereby combustion is generated by injecting oxidant and particulate liquid fuel, i.e. droplets of liquid fuel, into a combustion zone. The present invention relates more particularly to the use of such particulate liquid fuel burners for heavy liquid fuels.

2. Related Art

Coal is the most abundant fossil fuel currently available. Most of the power generated in the world uses coal as the fuel.

One way of generating heat or power is the use of coal burners.

In a coal burner, a conveying gas is often required to transport the solid fuel particles from a fuel storage or milling device (e.g. a coal pulveriser) to the burner for subsequent combustion with an oxidant. The oxidant for the combustion can be the conveying gas, a gas supplied separately from the conveying gas or a combination of the conveying gas and a separately supplied gas.

The combustion process of particulate solid fuel comprises several combustion steps which are described hereafter with reference to the combustion of particulate coal:

-   -   1) The coal particles, which typically leave the storage or         milling device substantially at ambient temperature, are heated         to at least the devolatilization temperature of the coal. The         devolatilization temperature is the minimum temperature at which         devolatilization of the coal commences (see hereafter). The         devolatilization temperature may vary depending on the type or         grade (class) of coal, its humidity, etc.     -   2) When the temperature of the coal particles reaches the         devolatilization temperature of the coal, devolatilization of         the coal particles commences. Devolatilization is a process in         which the volatile components (in short: volatiles) of the coal         particles leave the coal particles in gaseous form. Volatiles         include highly combustible components such as hydrocarbons and         hydrogen.     -   3) The volatiles combust with the oxidant, thereby generating         heat and increasing the temperature of the remaining solid         matter or char.     -   4) Finally, the char combusts with remaining oxidant, thereby         generating further heat.

This multi-step combustion process distinguishes particulate solid fuel combustion from the gaseous fuel combustion process in which the gaseous fuel combusts directly with the oxidant.

The particulate (i.e., atomized) liquid fuel combustion process, in which liquid fuel is injected into the combustion zone in the form of small particles or droplets, is also a multi-step process. In a first step, the injected liquid fuel droplets are heated to the evaporation temperature of the fuel when the fuel reaches its evaporation temperature, the liquid fuel evaporates to form inflammable fuel vapours and in the third step the inflammable fuel vapours combust with the oxidant and produce heat. For light fuels, such as domestic fuel oils, or No 1, 2 and 3 fuel oils, the evaporation temperature is relatively low and evaporation of the fuel into vapours takes place almost instantly following injection into the combustion zone at normal operational temperatures of most industrial furnaces. Consequently, the combustion of particulate light liquid fuels resembles that of gaseous fuels as far as rate of combustion following injection is concerned.

In the combustion of particulate medium heavy liquid fuels such as No 4 fuel oil and very heavy liquid fuels such as residual fuel oil, or No 5 and 6 fuel oils, the evaporation temperature is higher and evaporation of the liquid fuel takes place more slowly and more gradually. In this manner, the combustion of particulate heavy and especially very heavy liquid fuels resembles the multi-step process of particulate solid fuel combustion.

As a consequence, particulate solid fuel burners, such as particulate coal burners, and particulate heavy liquid fuel burners, are usually not suited for a narrow combustion chambers in which only short flames can be used for heat generation.

Indeed, when the length of the flame exceeds the width of the combustion chamber (the width being the free dimension of the combustion chamber along the flame axis), the flame impinges on the combustion chamber element opposite the burner, thereby causing incomplete fuel combustion and fouling with partial-combustion products such as soot as well as thermal damage to the impinged chamber element. The said chamber element can be a chamber wall positioned opposite the burner, for example in a glass feeder or forehearth or in a reheat furnace. The element can also be a chamber element to be heated such as a radiant heating panel or pipes, such as boiler pipes, positioned opposite the burner within the combustion chamber.

Air is traditionally used as the conveying gas and as the oxidant for particulate fuel burners, as the conveying gas for solid particulate fuels and as the pulverisation gas for particulate liquid fuel injectors. Burners using air as the oxidant for combustion are known as air-fuel burners.

In the case of oxy-fuel burners, the oxidant is an oxygen-rich gas (>25% vol O₂) such as oxygen-enriched air or industrial oxygen having an oxygen content of at least 90% vol, preferably of at least 95% vol, and more preferably of at least 98% vol.

The advantages of oxy-fuel burners over air-fuel burners are multiple:

-   -   improved fuel efficiency     -   improved heat transfer towards the charge to be heated,     -   reduced fumes generation,     -   higher CO₂ concentration in the fumes, which is advantageous for         CO₂ capture and sequestration     -   reduced pollutants emissions (e.g. NOx . . . ),     -   higher flame temperature providing improved the combustion of         hard-to-burn fuels,     -   etc.

In the case of oxy-fuel burners, the risk of thermal damage to the installation in case of narrow combustion chambers is particularly important due to the higher flame temperature when compared to air-fuel burners.

Examples of narrow combustion chambers are side-fired tunnel or passage furnaces, such as cement passage kilns, glass feeders or forehearths.

Other examples of narrow combustion chambers are side-fired vertical boilers and cracking installations (FCC).

In view of the high availability of solid fuels such as coal, including low-grade coal, and of heavy fuels, including some combustible industrial liquid wastes, often at advantageous prices, it would be highly desirable to be able to use particulate fuel burners in narrow industrial combustion chambers.

This is accomplished by the process of operating a furnace of the present invention.

SUMMARY OF THE INVENTION

The furnace used in said process comprises a combustion chamber defining a combustion zone within the combustion chamber. The combustion chamber has at least one chamber wall facing the combustion zone and at least one particulate fuel burner mounted in a chamber wall and adapted to generate a flame in the combustion zone by injecting oxidant in gaseous form and fuel in particulate form into the combustion zone for combustion therein.

According to the present invention, the at least one particulate fuel burner is operated so as to alternate between a first phase and a second phase.

During the first phase, the burner generates a wide flame in the combustion zone proximate the chamber wall into which the burner is mounted. During the second step, the burner generates a narrower flame, i.e. narrower than during the first phase, directed away from the chamber wall into which the burner is mounted.

In the present context, a wide flame is understood to mean a flame having an initial flame opening of at least 45°, preferably of at least 60°, more preferably of at least 80°, the flame opening being the cone angle α of the (truncated) cone defined by the root of the flame as shown in FIGS. 1 and 2. The narrower flame of the second step advantageously has a cone angle of less than 80°, preferably of less than 60° and more preferably of less than 45°.

The wide flame of the first step is generally also shorter than the narrower flame of the second step. The axis of the flame generated in the second phase is typically situated in a vertical plane perpendicular to the chamber wall in which the burner is mounted. For many applications, said flame axis is itself perpendicular to said chamber wall.

The furnace may comprise several such particulate fuel burners. In that case, each said particulate fuel burner is mounted in a chamber wall and adapted to generate a flame in the combustion zone by injecting oxidant in gaseous form and fuel in particulate form into the combustion zone for combustion therein. In accordance with the invention, each said particulate fuel burner is operated so as to alternate between the first phase and the second phase (as described above).

In a preferred method of operating such a multiple burner furnace, there is, at any moment during the process, a first portion of the particulate fuel burners which operate in the first phase, while the remaining portion of the particulate fuel burners operate in the second phase. In the present context, each “portion” comprises at least one particulate fuel burner. Naturally, as, in accordance with the present invention, the particulate fuel burners alternate between the first and the second phase, the same burner will at certain moments during the process belong to the first portion as defined above (i.e. operate in the first phase with a wide flame) and at other moments during the process to the remaining portion (i.e. operating in the second phase with a relatively narrower flame).

It will be appreciated that, in the present context, the phrase “at any moment during the process” refers to the steady operation of the process and excludes the starting and wind-down phases during which a different operation of the burners may be used, for example to preheat the furnace or to safely close the furnace down. It may be notably envisaged to operate all burners in the first phase mode (wide flame) during furnace preheating and start-up.

It is generally preferable that, at any moment during the process, at least half and preferably at least two thirds of the particulate fuel burners operate in the second phase (generating a narrower flame directed away from the chamber wall into which the burner is mounted). Indeed, as will be clarified below, the narrower flame may be more efficient in heating the charge.

According to one embodiment of the invention, a multitude of particulate fuel burners are mounted in one and the same chamber wall, each of said burners being adapted to generate a flame in the combustion zone by injecting oxidant in gaseous form and fuel in particulate form into the combustion zone for combustion therein. Said particulate fuel burners are operated so as to alternate between the first phase in which the burner generates a wide flame proximate said one chamber wall into the combustion zone and the second phase in which the burner generates a narrower flame directed away from said one chamber wall. Preferably, at any time during the method, a first portion of said multitude of particulate fuel burners mounted in said one chamber wall operate in the first phase, while the remaining portion of the multitude of particulate fuel burners mounted in said one chamber wall operate in the second phase.

When several particular fuel burners are mounted in one wall, these burners are usually arranged in a geometric pattern.

For example, a furnace with a long narrow combustion chamber may have a substantially horizontal row of burners mounted in one or both lateral chamber walls. A furnace with a vertical narrow combustion chamber may, for example, have burners arranged in a checkerboard pattern in a chamber wall.

According to an embodiment of the invention, a furnace may comprise one or more pairs of particulate fuel burners. Each such pair consists of a first particulate fuel burner and a second particulate fuel burner; whereby the second particulate fuel burner is positioned opposite the first particulate fuel burner across the combustion zone. When the first particulate fuel burner of a pair operates in the first phase, the second particulate fuel burner of the pair operates in the second phase. Similarly, when the first particulate fuel burner of a pair operates in the second phase, the second particulate fuel burner of the pair operates in the first phase. In this manner impact between the opposing flames can be avoided.

The present invention applies inter alia to furnaces having particulate fuel burners mounted in several chamber walls and in particular to furnaces having particulate fuel burners mounted in opposing walls.

The present invention thus relates in particular to a process of operating a furnace, whereby the furnace comprises a first multitude of particulate fuel burners mounted in a first chamber wall and a second multitude of particulate fuel burners mounted in a second chamber wall situated opposite the first chamber wall across the combustion zone. The particulate fuel burners are adapted to generate a flame in the combustion zone by injecting oxidant in gaseous form and fuel in particulate form into the combustion zone for combustion therein. They are operated so as to alternate between the first phase and the second phase. Preferably, at any time during the process, a first portion of the first and second multitude of particulate fuel burners operate in the first phase, while the remaining portion of the first and second multitude of particulate fuel burners operate in the second phase.

According to one embodiment of this process, at any moment during the process, the particulate fuel burners of the first multitude (which are mounted in the first chamber wall) simultaneously operate in either the first or the second phase and all the particulate fuel burners of the second multitude (which are mounted in the second chamber wall) likewise simultaneously operate in either the first or the second phase. In that case, it is preferable that:

-   -   when all the particulate fuel burners of the first multitude         simultaneously operate in the first phase, all the particulate         fuel burners of the second multitude, operate in the second         phase, and vice-versa.

In other words, when all of the particulate fuel burners mounted in the first wall operate in the first phase (wide flame proximate the first wall), all the particulate fuel burners mounted in the opposite wall operate in the second phase (narrower flame extending away from the second wall), and, when all of the particulate fuel burners mounted in the first wall operate in the second phase (narrower flame extending away from the first wall), all of the particulate fuel burners in the opposite wall operate in the first phase (wide flame proximate this wall).

As indicated above, this is particularly desirable when the individual burners of the first and second multitude are paired, i.e. when they are positioned one opposite the other.

The furnace may comprise several pairs of particulate fuel burners, whereby the first particulate fuel burner of each pair belongs to the first multitude of particulate fuel burners, and is therefore mounted in the first chamber wall, while the second particulate fuel burner of each pair belongs to the second multitude of particulate fuel burners, and is therefore mounted in the second chamber wall. The first and second particulate fuel burners of each pair are positioned opposite one another. As described earlier, when the first particulate fuel burner of a pair operates in the first phase, the second particulate fuel burner of the pair operates in the second phase. Likewise, when the first particulate fuel burner of the pair operates in the second phase, the second particulate fuel burner of the pair operates in the first phase.

As already described above, according to one embodiment of the invention, at any moment during the process, the particulate fuel burners of the first multitude simultaneously operate in either the first or the second phase and all the particulate fuel burners of the second multitude likewise simultaneously operate in either second or first phase. Similarly, according to one embodiment of the invention, the first particulate fuel burners of the pairs (which belong to the first multitude of particulate fuel burners) all simultaneously operate in either the first or the second phase and consequently all the second particulate fuel burners of the pairs (which belong to the second multitude of particulate fuel burners) all simultaneously operate in either the second phase (when the first burners operate in the first phase) or in the first phase (when the first burners operate in the second phase).

Such an operation of the furnace resembles the known alternating operation of side-fired furnaces, such as glass-melting furnaces, known in the art.

Preferably, however, at any moment during the process a first portion of the first particulate fuel burners (which belong to the first multitude of particulate fuel burners) operate in the first phase (while the second particulate fuel burners belonging to the same pairs operate in the second phase) and the remaining portion of first particulate fuel burners operate in the second phase (while the second particulate fuel burners of the corresponding pairs operate in the first phase).

For example, the first particulate fuel burners may be arranged in a row and may be operated so that, at any time during the process, said row consists of an alternating succession of burners operating in the first phase and burners operating in the second phase.

For many applications, the first and second walls are advantageously lateral walls of the combustion chamber. This is particularly the case for furnaces with long narrow cross-fired combustion chambers, such as glass feeders or forehearths, tunnel furnaces or kilns and reheat furnaces.

The fuel in particulate form can be a particulate solid fuel or a particulate liquid fuel.

Examples of suitable particulate solid fuel are particulate coal, pet coke, combustible particulate solid waste. Different classes of particulate coal may be used depending on the process: lignite, bituminous coal or anthracite, from highly coking to non-coking coals, etc.

Particular examples of suitable liquid fuels are medium heavy liquid fuels, such as No 3 fuel oil, and heavy liquid fuels such as No 5 and No 6 fuel oils, furnace fuel oils (FFO) and certain combustible liquid industrial wastes. The present invention is particularly useful for the combustion of waste fuel or fuel waste, if appropriate for the process concerned, in particular with regard to any effects on the charge to be heated.

Suitable particulate solid fuel burners, are notably known from WO200603296, WO2007063386, and from co-pending European patent application EP 09174622.2. Suitable particulate liquid fuel burners are known from EP 1750057 and WO03006879.

The particulate fuel burners may be oxy-fuel burners. In particular, the particulate fuel burners may be oxy-fuel burners operating with an oxidant, such as for example oxygen-enriched air, containing at least 50% by volume of oxygen, preferably at least 80% by volume, more preferable at least 90% by volume of oxygen, or industrial oxygen having an oxygen content of at least 95% by volume, and preferably of at least 98% by volume.

The particulate fuel burners preferably comprise a burner block defining a passage for the injection of fuel and/or oxidant therethrough into the combustion zone, the burner block being mounted in the chamber wall. The burner block is typically made out of refractory material, such as AZS.

The present invention is particularly useful for non-regenerative furnaces.

The duration of the first and second phase may be a predetermined duration. As will be explained below, the duration of the first phase is selected so as to be long enough to ensure a sufficiently high temperature in the combustion chamber proximate the particulate fuel burner so as to effectively shorten the flame obtained during the second phase, but not too long as to overheat and cause thermal damage to the burner or the chamber wall in the vicinity of the burner.

During the second phase, the temperature inside the combustion chamber proximate the burner decreases. The duration of the second phase is selected so as to switch back to the first phase before the temperature proximate the burner has dropped to such a level as to cause excessively long flames and/or incomplete combustion in the combustion zone.

For increased safety of operation, the furnace may be equipped with a temperature detector for detecting (a) the temperature in the combustion chamber proximate at least one particulate fuel burner, (b) the temperature of the furnace wall in the vicinity of the particulate fuel burner and/or (c) the temperature of the side of the burner block facing the combustion chamber when the particulate fuel burner comprises such a burner block. In that case, the duration of the first phase and of the second phase may be determined as follows:

-   -   the first phase of the particulate fuel burner is terminated and         the second phase is commenced when the first phase has reached         its predetermined duration or when the temperature detected by         the temperature detector reaches a predetermined upper limit and     -   the second phase is terminated and the first phase is commenced         when the second phase reaches its predetermined duration or the         temperature detected by the temperature detector reaches a         predetermined lower limit.

The predetermined upper limit is selected so as to prevent thermal damage to the burners or to the chamber wall on the side of the burners. The predetermined lower limit is selected so as to avoid excessively long flames, which could lead to incomplete fuel combustion and/or thermal damage to the chamber element opposite the burner.

The method of the present invention is particularly advantageous for narrow furnaces. The invention is notably advantageous for cross-fired narrow furnaces and in particular for tunnel furnaces. The benefits of the invention are particularly apparent when the furnace is a glass melting furnace, a glass feeder or forehearth, a tunnel calcination furnace or a reheat furnace. The present invention is also advantageous for narrow furnaces comprising burners on only one side of the combustion chamber, such as some vertical boiler furnaces.

Suitable particulate solid fuel burners, are notably known from WO200603296, WO2007063386, and from co-pending European patent application EP 09174622.2. Suitable particulate liquid fuel burners are known from EP 1750057 and WO03006879.

BRIEF DESCRIPTION OF THE FIGURES

The mechanisms and advantages of the methods according to the present invention are illustrated in more detail hereafter, reference being made to the non-limiting FIGS. 1 to 2 and to the examples, whereby:

FIG. 1 is a schematic partial cross section of a burner suitable for use in the process according to the invention operating in the first phase.

FIG. 2 is a schematic partial cross section of the burner of FIG. 1 operating in the second phase.

DETAILED DESCRIPTION OF THE INVENTION

In state-of-the-art combustion furnaces, the flame generated by said burners is generally directed away from the furnace wall towards the charge (for example, in the case of a vertical boiler furnace) or substantially parallel to the charge towards the opposite chamber wall (for example in a melting, fining or reheat furnace). In these state-of-the-art furnaces, a limited amount of heat is generated in the vicinity of the chamber walls. Indeed, high temperatures close to the chamber walls are generally avoided in order to prevent thermal damage to the refractory walls of the combustion chamber.

In the case of particulate fuel burners, and more specifically in the case of solid particulate fuel burners and particulate fuel burners for heavier liquid fuel fractions, this method of operating a furnace results in long flames and/or incomplete fuel combustion due to the long residence time/travel distance of the particulate fuel from its point of injection until it reaches its devolatilization/evaporation temperature. This makes these state-of-the-art furnaces badly suited for certain particulate solid and liquid fuels, in particular when the furnace is a narrow one.

The present invention now makes it possible to obtain relatively short particulate fuel flames by reducing the residence time required for the fuel particles to reach their devolatilization, respectively their evaporation temperature after their injection into the combustion zone, thus shortening the distance travelled by the fuel particles in the combustion zone before combustion of the volatiles, respectively of the fuel vapours commences, which in term leads to a shortening of the flame.

According to the present invention, this is achieved as follows.

The particulate fuel burners used in the process of the present invention are particulate fuel burners capable of varying the opening of the flame generated in a controlled manner.

Burners capable of varying the flame opening are known in the art. Examples of such burners are known from U.S. Pat. No. 3,337,324, WO-A-9744618 and WO-A-9627761. A particularly suitable burner for use in the method according to the present invention is disclosed in WO-A-2008003908.

In the particulate fuel burner shown by way of example in FIG. 1, the burner block 100, made of the refractory material AZS, comprises a through passage 200 into which are mounted, in concentric relationship, a central fuel injector 210 and a surrounding oxidant injector 220. Both injectors 210 and 220 are made of the heat resistant alloy Inconel® 600

The central fuel injector 210 may be a particulate solid fuel injector through which particulate solid fuel is transported by means of a conveyor gas. The central fuel injector 210 may also be a liquid fuel pulverizer, also known as “atomiser”, by means of which liquid fuel is sprayed into the combustion zone in the form of particulate liquid fuel. The mechanism of injecting particulate liquid fuel used by the central fuel injector 210 may be mechanical spraying, gas assisted spraying or a combination of the two. Suitable known spray devices are described in EP-A-1750057 and WO-A-03006879.

The surrounding oxidant injector is equipped with a swirling device 230 at its downstream end. This swirling device or “swirler” is capable of conferring a swirling movement, i.e. a rotational movement around the longitudinal axis of the injector to the oxidant injected into the combustion zone. The opening (α) of the flame generated increases with the swirling momentum conferred to the oxidant. Generally speaking, this also results in a shorter, but wider flame.

The oxidant used contains 90% vol O₂.

During the first phase of operating the particulate fuel burner, illustrated in FIG. 1, a wide flame 1 with flame opening α is generated, whereby the fuel combusts with the oxidant proximate the chamber wall 10 into which the burner is mounted. During the first phase, the wide flame 1 thus in particular heats the combustion zone proximate the burner and causes the temperature to rise in said area. During the first phase, the flame 1 may also cause an increase in the temperature of the burner block 100 and of the furnace wall 10 in the vicinity of the burner. The first phase is terminated before this increase in temperature leads to thermal damage of the burner/chamber wall. The maximum temperature to which the burner block 100 can be heated is determined by the thermal resistance properties of the burner block and of other burner constituent parts, such as metallic injectors mounted in or connected to said burner block. Fused cast AZS blocks have an important thermal inertia and high thermal resistance.

In order to prevent thermal damage to metallic parts of the burner block, these metallic parts are advantageously restricted to the rear half of the burner block (not shown), i.e. the metallic parts do not extend beyond half the width of the burner block starting from the surface of the block facing away from the combustion zone. A suitable choice of metal for (part of) the metallic parts can also help prevent thermal damage. Examples of such suitable metals are heat resistant alloys such as Inconel® 600 and Kantal®. The maximum temperature to which the chamber wall may be heated likewise depends on the material out of which said wall is made up.

During this first phase, heat transfer from the flame 1 to the charge to be heated (not shown) may be at less than optimum efficiency.

During the second phase, the particulate fuel burner generates a narrower (smaller flame opening a) and generally longer flame 1 for optimum heat transfer to the charge to be heated. In the embodiment illustrated in FIG. 2, this is achieved by reducing the amount of swirling momentum conferred by the swirler 230 to the oxidant flow or even by not conferring any swirling momentum to said oxidant flow (deactivated swirler 230).

As, in the preceding first phase, the combustion zone proximate the burner has been specifically heated, the temperature in said zone is higher than it would have been with the known method of operating a combustion furnace, so that the injected particulate fuel reaches its devolatilization/evaporation temperature more rapidly. As a consequence, the flame length is shortened and substantially complete combustion can be achieved, even in narrow furnaces.

If, during the first phase, the temperature of the burner block 100 has been made to increase, this may provide a further flame shortening effect. Indeed, during their passage through the burner block 100, the fuel and/or oxidant are heated by heat exchange with said burner block 100. The higher the temperature of the burner block 100, the more the fuel and/or oxidant are preheated and the quicker the particulate fuel reaches its devolatilization/evaporation temperature in the combustion zone 2.

During the second phase, the temperature of the combustion zone in proximity to the burner progressively decreases and the second phase is terminated and the first phase is started again no later than when the temperature of the combustion zone proximate the burner has dropped to such a level that the desired flame length/degree of combustion can no longer be achieved.

Example

A calcination furnace of the tunnel type for the production of clinker is equipped with a dozen pairs of particulate coal oxy-burners. Each lateral wall of the furnace (taken in the direction of travel of the product to be calcined) presents a substantially horizontal row of particulate fuel burners. The burners in the first lateral wall are positioned in staggered relationship with the burners in the second opposite wall. The burner blocks are made of AZS. The metal reactant injectors are made of the heat resistant alloy Inconel® 600 and are positioned in the rear three quarters of the burner blocks.

Tests were conducted with particulate liquid fuel known as heavy fuel FO 2 and with particulate bituminous coal.

1) One-Phase Operation of First Burners Only (not in Accordance with the Invention).

In a first test, all particulate fuel burners were operated at nominal power to generate a flame in the combustion zone situated between the lateral walls.

The flames so generated clearly impacted the lateral wall on the opposite side of the combustion chamber causing rapid overheating thereof. The test was terminated before the lateral chamber walls suffered thermal damage.

2) Two-Phase Operation According to a First Embodiment of the Invention.

In a second test, the furnace was operated according to the embodiment of the invention whereby all the particulate fuel burners in one lateral chamber wall all operate in the same phase and whereby the burners in the opposite lateral chamber wall all operate in the other phase.

Thus, the furnace alternates between (a) all burners in the first lateral wall operating in the first phase (wide flames proximate the first lateral wall) and all burners in the second lateral wall operating in the second phase (narrow long flame across the combustion zone), and (b) all burners in the second wall operating in the first phase (wide flame proximate the second wall), and all burners in the first wall operating in the second phase (narrow long flames across the combustion zone).

At nominal burner power, the flames generated by the burners in the second phase were found not to impact the lateral wall opposite the burners. No significant deposition of partial combustion products (soot) on the opposite chamber wall was observed and no thermal damage to the chamber wall was detected. The energy efficiency of the furnace was found to be at least equivalent to that obtained in the first test, and generally better.

3) Two-Phase Operation in Accordance with a Second Embodiment of the Invention.

In a third test, the furnace was operated according to the embodiment of the invention whereby the successive particulate fuel burners in each row alternate between burners in the first phase and burners in the second phase. The burners were operated at nominal burner power.

The results obtained were similar to those obtained in the second test with the added advantages that the average temperature profile perpendicular to the direction of flow flow of the product was symmetrical and remained substantially constant over time.

It will be understood that many additional changes in the details, materials, steps and arrangement of parts, which have been herein described in order to explain the nature of the invention, may be made by those skilled in the art within the principle and scope of the invention as expressed in the appended claims. Thus, the present invention is not intended to be limited to the specific embodiments in the examples given above. 

1-16. (canceled)
 17. A process of operating a combustion furnace, comprising the steps of: providing a combustion furnace comprising: a combustion chamber defining a combustion zone within the combustion chamber and having at least one chamber wall facing the combustion zone, and at least one particulate fuel burner mounted in a chamber wall and adapted to generate a flame in the combustion zone by injecting oxidant in gaseous form and fuel in particulate form into the combustion zone for combustion therein; and performing alternating first and second steps, wherein: in a first phase, at least one of the at least one particulate fuel burner is operated to generate a wide flame proximate the chamber wall into which it is mounted, and in a second phase, the at least one of the at least one particulate fuel burner is operated to generate a narrower flame directed away from the chamber wall into which it is mounted.
 18. The process of claim 17, wherein the furnace comprises several particulate fuel burners, each mounted in a chamber wall and adapted to generate a flame in the combustion zone by injecting oxidant in gaseous form and fuel in particulate form into the combustion zone for combustion therein, each particulate fuel burner being operated so as to alternate between the first phase and the second phase and wherein at any time during the process a first portion of the particulate fuel burners operate in the first phase, while the remaining portion of the particulate fuel burners operate in the second phase.
 19. The process of claim 18, wherein at any time during the process at least half of the particulate fuel burners operate in the second phase.
 20. The process of claim 19, wherein at any time during the process at least two thirds of the particulate fuel burners operate in the second phase.
 21. The process of claim 18, wherein the furnace comprises a multitude of particulate fuel burners mounted in one chamber wall and adapted to generate a flame in the combustion zone by injecting oxidant in gaseous form and fuel in particulate form into the combustion zone for combustion therein, each particulate fuel burner of the multitude being operated so as to alternate between the first phase and the second phase and wherein at any time during the process a first portion of the multitude of particulate fuel burners mounted in the one chamber wall operate in the first phase, while the remaining portion of the multitude of particulate fuel burners mounted in the one chamber wall operate in the second phase.
 22. The process of claim 18, wherein the furnace comprises one or more pairs of particulate fuel burners, each pair consisting of a first particulate fuel burner and a second particulate fuel burner, the second particulate fuel burner being positioned opposite the first particulate fuel burner and when the first particulate fuel burner of the pair operates in the first phase, the second particulate fuel burner of the pair operates in the second phase and, when the first particulate fuel burner of the pair operates in the second phase, the second particulate fuel burner of the pair operates in the first phase.
 23. The process of claim 18, wherein the furnace comprises a first multitude of particulate fuel burners mounted in a first chamber wall and a second multitude of particulate fuel burners mounted in a second chamber wall opposite the first chamber wall, the particulate fuel burners being adapted to generate a flame in the combustion zone by injecting oxidant in gaseous form and fuel in particulate form into the combustion zone for combustion therein, and being operated so as to alternate between the first phase and the second phase.
 24. The process of claim 23, wherein, at any moment during the process: either all the particulate fuel burners of the first multitude simultaneously operate in the first phase while all the particulate fuel burners of the second multitude operate in the second phase, or all the particulate fuel burners of the first multitude simultaneously operate in the second phase while all the particulate fuel burners of the second multitude simultaneously operate in the first phase.
 25. The process of claim 23, wherein the furnace comprises several pairs of particulate fuel burners, each pair consisting of a first particulate fuel burner belonging to the first multitude of particulate fuel burners and a second particulate fuel burner belonging to the second multitude of particulate fuel burners, the first and second particulate fuel burners of each pair being positioned opposite one another, wherein: when the first particulate fuel burner of a pair operates in the first phase, the second particulate fuel burner of the pair operates in the second phase; and when the first particulate fuel burner of the pair operates in the second phase, the second particulate fuel burner of the pair operates in the first phase.
 26. The process of claim 25, wherein, at any time during the method, a first portion of the first particulate fuel burners operate in the first phase and the remaining first particulate fuel burners operate in the second phase.
 27. The process of claim 23, wherein the first and second walls are lateral walls of the combustion chamber.
 28. The process of claim 25, wherein the first and second walls are lateral walls of the combustion chamber.
 29. The process of claim 17, wherein the fuel injected by the particulate fuel burners is a particulate solid fuel.
 30. The process of claim 17, wherein the fuel injected by the particulate fuel burners is a particulate liquid fuel.
 31. The process of claim 17, wherein the particulate fuel burners are oxy-fuel burners.
 32. The process of claim 17, wherein the furnace is a glass-melting furnace, a glass feeder or forehearth, a tunnel calcination furnace or a reheat furnace. 